This invention relates to a picture coding and/or decoding equipment which is particularly suitable for coding and/or decoding of a color still picture.
For efficient transmission and storage of pictures, compressed encoding has been proposed. As conventional methods for coding a color picture, there are known the .DELTA.PCM coding and the DPCM coding method for transmitting a motion picture. These prior art methods are to separate a color picture signal into three color components Y, I and Q and process them individually. The .DELTA.PCM coding method is one that is to sample a television signal by a suitable clock of, for example, about 9 to 10 MHz, compare the signal level at each sample point with the signal level at the immediately preceding sample point and encode the difference therebetween as a PCM code. The DPCM coding method is one that is to predict the signal level at each sample point using a pel (picture element) immediately preceding it and the value of a pel lying on the immediately preceding scanning line right above it and encode the difference between the predicted value and the actual signal level. Such coding methods are employed for the processing of motion pictures, and the coding of one frame must be accomplished within 33 ms which is the transmission time of one frame, and hence the processing is limited to simple one; furthermore, the amount of coded data for one frame must be retained substantially constant. Moreover, in either case, if the number of quantization levels of the differential signal is extremely reduced, then the picture quality is degraded by an overload noise, granular noise and so forth; this imposes limitations on high compressed encoding of a picture.
As a coding method for a still color picture, transfer picture coding is known, as set forth in K. PRAIT "Slant Transform Image Coding", IEEE, Trans. on. Com., Vol. COM-22, No. 8, 1974, 8, pp 1075-1093. With this method, a color picture is separated into color planes Y, I and Q; each color plane is divided into blocks, each including about 16.times.16 pels; the blocks are each subjected to cosine transformation or slant transformation; and according to the transformation result, the blocks are classified in dependence on their internal measure of activity. Encoding is performed changing the number of allocated bits and quantizing characteristics for each class. In this way, high compressed encoding to 1.about.2 bits per pel is carried out. With this conventional method, a two-dimensional picture is transformed into a frequency space and only its low-frequency component is returned to a real space, so that orthogonal transformation takes place twice. But the orthogonal transformation operation is very complicated and involves a large amount of data processed, and in addition, the decoding operation is also complicated.
It is an object of this invention to provide a picture coding and/or decoding equipment which is capable of encoding a color picture with a high compression ratio without involving complicated processing.
Another object of this invention is to provide a picture coding and/or decoding equipment which is capable of encoding a color picture with a high compression ratio through utilization of color characteristics of the picture.